1. Field of the Invention
This invention relates to a cable shield electrical connector used one on each side of a communications cable splice to provide electrical continuity of a cable shield across the splice as well as to mechanically connect the shield of a cable to the shields of secondary cables, grounded service wires or other grounding devices.
2. Description of the Prior Art
Telephone cable systems normally include a plurality of discrete cable lengths which are joined together at splice locations or which are joined to other apparatus at terminal points. Each of these discrete cable lengths comprises a multi-conductor core which is enclosed in a metallic shield and an outer plastic sheath. The electrical shield normally takes the form of a polyethylene-coated, corrugated cylinder, usually a good conductor such as aluminum, which forms a tubular member interposed between the conductors and the cable sheath.
A metallic shield in a telephone cable performs a variety of important functions. Some of these are protection of installers from injury and equipment from damage if a live power line should contact the cable, protection from induced current from power lines, protection from currents resulting from lighting, and suppression of radio frequency interference. The metallic shield also provides physical protection of the cable core and acts as a barrier to moisture penetration.
To obtain effective shielding from power line induced noise, for example, shield continuity and earth grounding must be provided throughout the cable. At splice locations, where the cable sheath and shield are removed to expose the individual conductors, it is necessary to provide for continuity of the shield across the splice for proper electrical protection of the conductors. Moreover, it is necessary that the cable shield be earth grounded. Connection to the cable shield at splice locations and its terminal ends is generally accomplished with a shield connector which may be referred to in the art as a bond clamp or bonding connector.
Investigation of many presently available cable shield connectors reveals that contact resistance between the connector and the shield increases substantially with time and, as a result, telephone companies have experienced noisy lines. The increase in contact resistance has been attributed to loss of contact between the connector and the shield, which results in oxidation of the shield at the contact points. Aluminum, as well as the cable sheath, which is normally a low density polyethylene, have the tendency to relax by cold flow or creep under sustained load, and, in addition, the dimensional stability of the sheath is very sensitive to temperature fluctuations. Therefore, dissipation of the initially applied pressure at the contact points between the connector and shield takes place with time and aluminum oxide forms which is non-conductive and consequently results in increased electrical resistance between the connector and the shield.
This difficulty in achieving adequate electrical contact to the cable shield has recently been complicated by the provision of a secondary, polyethylene-coated steel shield between the aluminum shield and the cable sheath. This cable is known as a coated aluminum, coated steel, polyethylene (CACSP) cable, with the steel shield being provided primarily to protect the cable from physical damage. It is, however, required that continuity of the steel shield be maintained and that the steel shield be electrically connected to the aluminum shield at splice points to ensure that a voltage potential never exists between the two shields and to provide an additional conductive path for high amperage currents such as results from lightning strikes.
Some shield connectors have an inherent spring reserve to press the contact elements together and compensate for cold flow in the shield and the sheath, thereby minimizing the increase in contact resistance with age. These connectors generally fall into two categories, which include the cantilever types and the direct force types. The cantilever types have pivoting top and bottom plates capturing an end portion of the sheath and shield which are pulled together by joining means, usually a bolt, external to the contact area, i.e., between the pivot and the contact area. Examples are the connectors of U.S. Pat. Nos. 3,778,749 and 3,787,797. The direct force types are the most common and include a centrally located joining means pulling top and bottom plates together in the contact area. In this case the joining means passes through a hole or slit in the sheath. Examples are the connectors of U.S. Pat. Nos. 3,676,836 and 3,701,839.
The cantilever type of connector has the advantage of a potentially large travel and spring reserve. Its primary disadvantage is lower initial contact force, typically one-half the tension in the joining means. The direct force type of connector provides initial contact force approximately equal to the tension in the joining means, but it has a small potential travel stored in the resiliency of the connector and, therefore, does not compensate for cold flow of the cable sheath very well.
In view of these problems with existing shield connectors, design criteria for an improved shield connector might include high initial contact force together with a long travel provided by a spring reserve which maintains low electrical contact resistance independent of cold flow of the sheath material.
In addition to failures caused by increased contact resistance, fault currents and lightning surge currents also have been known to cause shield connector failures by melting the joining means, usually a threaded stud, pulling the top and bottom plates together in the contact area. Another design criteria for an improved shield connector is that the connector should be highly resistant to these damaging currents.
Finally, an improved shield connector must provide a strong mechanical grip on the cable and be resistant to forces which would tend to disturb contact integrity or pull the connector free of the cable shield and sheath.